Self assessment - you're on your Jack Jones.

Look up self assessment in Google and "calculation of one's own tax liability" pops up and, to be fair to HMRC, when it was introduced in April 1996 I don't think it was intended to stretch much beyond this.

Huge cuts to the the HMRC workforce in the last 20 years have meant that "calculation of one's own tax liability" is only the first step in making sure your tax is correct.

A combination by HMRC of "automation", such as the issue of unchecked PAYE Coding Notices, and the failure to review the figures on the tax return has resulted in far too many incorrect liabilities slipping through the system.

And before you start saying that the error is in your favour don't forget that HMRC will ask for the underpaid tax to be paid with penalties on top of the tax.

My advice, not surprisingly, to avoid this situation is to make sure you have an accountant competing your tax return. Not just any accountant though, one who sweats the small stuff, to make sure you do pay the correct amount of tax.

Days to January 31st